In the three-dimensional EDM process with at least one generic tool electrode having a machining surface contour at an end portion thereof, the tool electrode is axially juxtaposed with a workpiece to position the machining surface contour in spaced juxtaposition therewith across an EDM gap supplied with a machining liquid. A succession of electrical discharges are produced across the EDM gap to electroerosively remove stock from a localized portion of the workpiece juxtaposed with the machining surface contour. To advance the process, the workpiece is displaced relative to the tool electrode along a three-dimensional path, typically under numerical control, while the width of the EDM gap is held substantially constant whereby the desired cavity or contour dissimilar to the generic electrode and basically determined by the path of the three-dimensional feed displacement effected between the tool electrode and the workpiece is carved out in the workpiece.
The advantages of the generic electrode EDM process over the conventional "sinking" EDM process which makes it essential to use several similar formed electrodes of mirror images of a desired cavity or contour are being increasingly recognized in the art. In the sinking EDM process, it has been found to be very difficult to prepare a formed tool electrode of a precise mirror image of a desired cavity or contour. In addition, several such electrodes of slightly varying sizes are required to allow repetition of the process in different modes ranging from roughing to finishing. Because of these electrode factors the sinking EDM process for machining a three-dimensional cavity or contour has been very costly and laborious. There is also the problem of electrode positioning at successive stages of electrode exchange. It has also be recognized that as the machining depth increases the rate of stock removal decreases and this necessitates a sophisticated power supply, and complex controls of machining feed, dielectric circulation and electrode reciprocation.
By contrast, in the generic EDM process a simple tool electrode in the form of a cylinder of small cross section or the like, or more than one such simple electrode varying in size can simply be employed to machine a large and/or intricate cavity or contour. The cavity or contour is easily machined in the workpiece by displacing the generic electrode and the workpiece relative to each other, under numerical or sequence-copying control, along a prescribed three-dimensional path which determines the final cavity or contour desired in the workpiece. Since the generic electrode is allowed to move generally in an open space to advance machining, the process does not present a problem as is encountered by sinking EDM as the depth increases.
While the generic EDM process offers these particular advantages, it has now been found that its "open space" feature presents a particular problem. Thus, due to the fact that the active machining surface of the electrode is much smaller than the area of the workpiece traversed thereby, chips and gases produced by the electrical discharges are removed rather quickly from the EDM gap. As a result, the concentration of these machining products in the machining liquid at the EDM gap remains at an undesirably low level. The inventor's experimentation shows that for stock removal electrical discharges to be produced consecutively or with stability, the machining liquid must be contaminated with the machining products to a certain degree. Thus, in the process, while abnormal electrical discharges due to an excessive accumulation of the machining products in the gap seldom occur, it has been found that the machining electrical discharges tend to be destabilized or fail to occur consecutively. This may result in an unstable machining feed displacement and unsatisfactory machining performance.
For example, utilizing a kerosene machining liquid, under a no-load pulse voltage of 100 volts, an electrical discharge is not created until the machining gap between the generic electrode and the workpiece is narrowed to as small as 3 .mu.m. By comparison, in the sinking EDM process a gap spacing of as wide as 39 .mu.m is satisfactory to allow successive, time-spaced electrical discharges to be created consecutively or with a rate of occurrence of electrical discharges per applied voltage pulses ranging from 40 90%. This evidently is due to the fact that machining products produced by previous discharges are contained at a significant proportion in the machining liquid and facilitate the production of subsequent electrical discharges.
Assume that by means of a servo feed, the machining gap is narrowed to a spacing of 3 .mu.m to allow an electrical discharge to occur followed by several electrical discharges due to the consecutive application of voltage pulses. The servo system will then find the gap spacing of 3 .mu.m too small and act to widen the machining gap. Due to inertia inherent in the servo system, the gap is then widened to an excessive extent (e.g. 50 to 60 .mu.m) such that an electrical discharge may no longer occur. The servo system will then act on the generic electrode to narrow the gap. At this stage the machining products will have been substantially completely carried out to clear the machining gap because of the open-space arrangement of the generic electrode. Thus, the machining gap will have to be reduced to as narrow a spacing as the previous spacing of 3 .mu.m. The result of repetitions of such a cycle is so-called "hunting" which allows no substantial or only an extremely limited stock removal from the workpiece.